In the load balancing system, a large amount of processing requests issued from client devices can be processed at high speed by being distributed among a plurality of servers. In recent years, a load balancing system having a hierarchical structure with a plurality of servers connected hierarchically has been used for the purpose of preventing the reduction in communication speed. FIG. 1 is a diagram schematically depicting a configuration of a load balancing system having the hierarchical structure with the servers connected hierarchically.
The hierarchical load balancing system 1, which can be illustrated as a network with nodes 111 to 113, 121, 131 to 139, 141 to 143 and 151 to 159 connected in hierarchy. The load balancing system 1 includes various devices such as servers having the functions of processing, distributing and transferring the processing requests, or firewalls, as the above nodes 111 to 113, 121, 131 to 139, 141 to 143 and 151 to 159. The node units 111 to 113 in the highest layer called the front end, upon reception of processing requests from client devices (hereinafter referred to simply as “the clients”) C1 and C2 through a network N, transfer these processing requests sequentially from the nodes in the high layers to the nodes in low layers. While proceeding to the tail end, the load balancing nodes 121 and 141 to 143 distribute the processing requests among a plurality of nodes so that the load on each node is reduced by distribution.
Incidentally, each of the load balancing nodes 121 and 141 to 143 may be a load balancer, a load balancing application program or a load balancing middleware executed on and causing these servers to execute the load balancing process. Also, the node units 111 to 113 in the highest layer are not limited to the server units but may be any of various devices such as the load balancers, the firewall devices or the load balancers having the firewall function. As described above, the hierarchical load balancing system 1 can be implemented with various configurations. In this specification, an example of a specified configuration is used solely for the convenience of illustration. The embodiment, however, is not limited to this specified configuration, but is applicable also to any hierarchical load balancing system having various configurations.
FIG. 2 is a diagram depicting an example of the configuration of the conventional hierarchical load balancing system. In this load balancing system 1, the servers 11 to 13 in the highest layer (first layer) accept the processing requests given by the clients C1 and C2, and these processing requests are transmitted to a load balancer (LB) 21 arranged in the next lower layer (second layer). In the load balancer 21, each processing request received from the servers 11 to 13 is distributively transmitted to the servers 31 to 39 arranged in the next lower layer (third layer) in accordance with the manner corresponding to the situation of the various load balancing manners. The load balancing manners in general use include, in addition to the least connection and the round robin, a method for determining the distributee according to the value of the hash function of the address of the transmittee or the transmitter of the processing request (referred to as “the hash method” hereinafter in this specification), a method for determining the distributes in accordance with the HTTP header of the processing request (referred to as “the HTTP header method” hereinafter in this specification) and a weighting method.
The servers 31 to 33 arranged in the third layer transmit the processing requests received from the load balancer 21 to the load balancer 41 arranged in the next lower layer (fourth layer). In similar fashion, the servers 34 to 36 transmit the processing requests received from the load balancer 21 to the load balancer 42. Also, the servers 37 to 39 transmit the processing requests received from the load balancer 21 to the load balancer 43. The load balancer 41 arranged in the fourth layer distributes the processing requests received from the servers 31 to 33 to the servers 51 to 53 in the lowest layer (fifth layer) in accordance with a manner corresponding to the prevailing situation, of all the various load balancing manners. In similar fashion, the load balancer 42 distributes the processing requests received from the servers 34 to 36 to the servers 54 to 56, and the load balancer 43 distributes the processing requests received from the servers 37 to 39 to the servers 57 to 59.    Patent Document 1: Japanese Unexamined Patent Publication No. 2003-196178